A soldering flux is used to aid fusion of the parts being joined, by reacting with and dissolving their surface oxides or impurities while coating the surfaces against oxidation, during soldering at a temperature above the solder melting point.
Organic fluxes are typically based on water-insoluble rosin or water-soluble organic acid. Activated rosin fluxes are used in soldering electrical connections on printed circuit boards. Wave soldering is used for mass production circuit board soldering as by applying the flux, preheating the board, applying the solder (e.g., via a molten solder standing wave), cooling the board, and cleaning it to remove flux residue.
Such flux residue is mainly constituted of ionic (e.g., acidic or basic) substances, and is corrosive, or can hydrolyze to corrosive constituents in the presence of moisture (e.g., air moisture). This can lead to short circuits, noise generation, etc., in use of the circuit board product. These adverse results are effectively avoided by subjecting the soldered board to a cleaning step to remove the ionic substances.
U.S. Pat. No. 2,715,084 to Konig shows a flux of rosin activated by a chain-brominated higher aliphatic monocarboxylic acid or salt of 8-24 chain carbon atoms (e.g., alpha-bromolauric acid), in turpentine, that does not decompose at the soldering temperature. It is non-corrosive as the chain bromine is not ionic so that the compound does not hydrolyze to corrosive halide, as with acid or halide salt flux, or rosin flux activated by organic base halide (e.g., cetyl-pyridinium halide), wherein the halide as an anion can hydrolyze in air to corrosive acid halide. Brominated acids of less than 8 carbon atoms are excluded as being corrosive and of less surface activity.
U.S. Pat. No. 2,898,255 to Thompson et al. shows a non-corrosive printed circuit board flux of rosin activated by a combination of both an aliphatic monocarboxylic acid (e.g., formic, acetic or propionic acid) and an aliphatic dicarboxylic acid (e.g., oxalic, malonic, succinic, glutaric or adipic acid), in isopropanol, that does not splatter during soldering. The sole example given of a non-splattering flux is a mixture of rosin (mp 120.degree. C./248.degree. F.), formic acid (bp 100.degree. C./212.5.degree. F.) and glutaric acid (bp 200.degree. C./392.degree. F.), in isopropyl alcohol, in which the low glutaric acid content (about 1%) is said to prevent splattering, while the given melting and boiling points of the components are said to permit step-by-step deoxidation of the oxide surface being soldered.
U.S. Pat. No. 2,904,459 to Kubota shows a liquid, paste or solid flux for electrical application. The flux is based on rosin converted to pure, stable abietic acid amine salt (e.g., diethylene amine salt), that is insoluble and does not hydrolyze, and whose neutral resinous residue on soldering is non-corrosive and non-hygroscopic, unlike commercial rosin which, besides a 25-30% content of abietic acid (i.e., a monobasic organic acid), that has fluxing action, also has a resinous remainder that can cause soldered product defects.
U.S. Pat. Nos. 3,235,414 and 3,264,146 to Marks show a rosin-free food can soldering flux. It is formed as a relatively concentrated solution of a large percentage (e.g., 25 wt. parts) of an aliphatic dicarboxylic acid of 9-10 carbon atoms with at least 4 carbon atoms in the chain separating the carboxyl groups (e.g., azelaic or sebacic acid), or its anhydride or ester, that is modified by (e.g., 1.2 wt. parts) ethanolamine hydrochloride, in (e.g., 75 wt. parts) dimethylformamide as polar solvent. Such acids with 3 or less separating carbon atoms are undesired in forming monoanhydrides from a single molecule.
U.S. Pat. No. 3,837,932 to Aronberg shows a 10-80% polycarboxylic acid particulate suspension (e.g., adipic acid), in an aqueous solution of 0.2-5% soluble gum (remainder 15-89.8% water), as rosin-free soldering flux for bonding lead storage battery plate lugs to straps, in which the acid (e.g., adipic) does not sublime at the high (482.degree. C./900.degree. F.) bonding temperature. Use of the dry particulate acid alone, applied to the lugs after preheating, is less preferred.
U.S. Pat. No. 4,168,996 to Zado shows a mixture of rosin, an activator with both a halogen and a destabilizing substituent, such as a halogenated mono or dibasic organic acid (e.g., 2,3-dibromosuccinic acid), and a solder surfactant such as a polycarboxylic acid (e.g., adipic, sebacic or azelaic acid), or hydroxyl substituted polybasic acid, and/or keto acid (e.g., levulinic acid), in ethanol or isopropyl alcohol, and optionally a secondary activator (e.g., diethylamine hydrochloride) and a non-ionic foaming agent, as activated rosin flux of synergistic effect for printed circuit boards.
Zado notes that using rosin with the activator or solder surfactant alone does not provide the synergistic effect, and that the combination acid containing rosin flux of the above-discussed Thompson patent is too acidic and corrosive for practical use in electronics soldering.
The importance of a residue-free soldered connection is discussed in the article entitled: "Europeans find ways to phase-out CFCs," Electronic Packaging & Production, Vol. 29, No. 1, January 1989, pp. 26 and 28, Linda Smith-Vargo, Associate Editor. The article notes that fluxes used in printed circuit board applications leave residues that must be cleaned, typically by an organic liquid based on fully halogenated chlorofluorocarbons or CFCs, but that in view of the need to reduce CFC emissions, due to environmental constraints, soldering procedures are needed that avoid CFCs yet produce a residue-free non-corrosive product.
It is desirable to have a flux for soldering metal connections in electrical applications such as printed circuit boards that does not produce a corrosion-causing and/or hydrolyzable ionic residue that requires a cleaning step for its removal.